REducing aViation Emissions’ uncertAin cLimate impacts: NOx
“The chain of events from emissions of NOx to impacts on climate are complex and uncertain. Are these aviation NOx emissions responsible for a lot or a little warming or cooling? In this project we aim to constrain the uncertainty we have on these climate impacts by constraining the performance of models in representing the chemical cascades from aviation NOx emissions” - Professor Alex Archibald, speaking to NCAS Comms
This document provides a summary of the REVEAL-NOx flights. It is primarily an overview of the flights and a discussion on processing the NO and NO2 data. It highlights other notable considerations, such as periods where flights passed through stratospheric air masses and biomass burning plumes. It also provides a brief overview of the VOC data from the SWAS canisters.
The code used to produce this document can be accessed on github, and the merged FAAM data can also be downloaded here. Efforts will be made to keep the merge up to date, but the versions on CEDA should always be considered the authoritative copies. The github repo includes code for downloading the data directly from CEDA using the faamr package.
Overview
Eight flights aboard the FAAM aircraft were conducted between 2025/06/03 and 2025/06/12 (C409 - C418) following roughly the same flight plan, either clockwise or counter-clockwise.
The counter-clockwise case:
- Take off, heading east and climb to flight level 300
- Fly a circuit of off the East of England, following flight lanes L603 and L602
- Turn north and fly towards Aberdeen along the east coast of the UK (Transect Coast)
- Just south of Aberdeen, turn south and ascend to flight level 340
- Fly due south, roughly tracing the 2° west line of longitude until south of Birmingham (Transect Land)
- Recover to Cranfield
All flights were flown as doubles, with the direction reversing in the afternoon, such that the circuit and 2° W transect were flown at both FL300 and FL340 each day. Flight tracks can be viewed below, coloured by altitude or region, for the latter only level runs above 9000 m are shown.
Here time series of key measurements can be viewed for each flight.
NOx processing details
Data + Cleaning
- All data was used, including those flagged
Reduced QualityorSuspectdue to the lack of clear definition of these terms - Apply 10-6 - 106 pptv threshold for spike removal
The Limit of Detection (LOD)
- The 10 Hz NOx data is often close the the limit of detection, especially when the uncertainty is considered
- Time averaging improves this, here we average to 10 s, reducing the uncertainty \(\sqrt{\frac{\sum{u^2}}{N}}\) and LOD by \(\sqrt{\frac{\sum{LOD}}{N}}\)
The below plot demonstrates this on two 500 s periods taken from C409, data are red when the mixing ratio - uncertainty are less that the LOD (shown as a black line). The y-axis is pseudo-log10 1.
Move the slider to switch between the 10 Hz and 10 s data.
Data recovery is much improved when treating the LOD in this fashion. The below plots shows 10 s data for all flights
Data by Region and Altitude
For quick examination, plotted below are the densities of NO and NO2 mixing across all of the REVEAL flights after the processing described above. Altitude has been rounded to the nearest 1000 m as a rough grouping. Enhanced NOx is generally seen as altitude increases, and this is consistent with the expectation that FL300 was not quite high enough to see the effects of the jetways. The non-rounded altitude data is shown in the footnotes 2.
Stratospheric Intrusion - Evidence from O3, CH4, and RH
Particularly in flights during the first week, (C409 - C413), there are periods where the aircraft flew through areas of stratospheric intrusion into the free troposphere. These are indicated by periods of elevated O3 and suppressed CH4 and RH. As a broad filter, periods where O3 > 300 ppbv and RH < 20 % helps identify these. During these CH4 tends to be < 1900 ppbv, though this threshold is less clear cut (see C409 and C410).
The below plots show these parameters as well at altitude and NOx across all flights. C413 provides an excellent example of clear flight in the stratosphere, with the entire 1000 m leg meeting the above criteria, and the transition from free troposphere being evident as the aircraft changes height from 9000 m. Periods of the flight < 6000 m have been removed.
